A magnetoresistive effect type magnetic head may be employed as a sensor for reproduction of magnetic information recorded on a magnetic recording medium in high recording density magnetic recording devices, such as a high density hard disk, and is the component that largely governs performance in magnetic recording techniques.
In recent years, magnetic reproduction heads are being employed that use, for example, the magnetoresistive effect of a multilayer film comprised by laminating ferromagnetic metallic layers with non-magnetic intermediate layers therebetween. This is called giant magnetoresistive effect (GMR). For such GMR heads, originally, Current-in-Plane (CIA) type heads were employed, in which the sensor current flows in a parallel fashion within the plane of the sensor film. In order to improve recording density, tunneling magnetoresistive effect (TMR) heads and Current-Perpendicular-to-Plane-Giant Magnetoresistive effect (CPP-GMR) heads have been developed and used, which are beneficial in achieving higher output with reduced track width and reduced gap width, and TMR heads now represent a majority of magnetic reproduction heads being produced. Such TMR heads and CPP-GMR heads are CPP type heads in which the sensor current flows in perpendicular fashion within the sensor film, in contrast to conventional GMR heads.
When the giant magnetoresistive effect and tunneling magnetoresistive effect are employed in a magnetoresistive element, a structure called a spin valve may be used. A spin valve typically has a laminated structure comprising an antiferromagnetic layer/ferromagnetic layer/non-magnetic intermediate layer/ferromagnetic layer. The magnetization of the ferromagnetic layer that is in contact with the antiferromagnetic layer is substantially fixed by the exchange coupling magnetic field generated at the antiferromagnetic layer/ferromagnetic layer interface. Output is obtained by free rotation of the magnetization of the other ferromagnetic layer by an external magnetic field. The ferromagnetic layer whose magnetization is substantially fixed by the antiferromagnetic layer is called the fixed layer, while the ferromagnetic layer whose magnetization is rotated by an external magnetic field is called the free layer. The reproduction output is generated as the product of the utilization rate with the magnetoresistive (MR) ratio, which is the rate of change of resistance produced by the drive voltage and the magnetoresistive effect. The utilization rate is an index indicating to what extent the free layer magnetization is rotated by the magnetic field applied from the magnetic recording medium. A high utilization rate means high output; but, if the utilization rate is too high, the change in resistance with respect to the magnetic field becomes non-linear, with the result being that performance of the magnetic recording/reproduction device is adversely affected. Usually, for this reason, a utilization rate of about 20% to 30% is set. Typically, the magnitude of the utilization rate is appropriately controlled by optimizing, for example, the material of the domain control layer provided on both sides of the track direction of the multilayer film of the magnetoresistive effect head, and/or the film thickness.
Typically the MR ratio is raised in order to increase the reproduction output, which largely governs the performance of the magnetoresistive effect head. TMR heads, as referred to above, are currently widely employed, in order to achieve the highest possible MR ratio.
Another factor that largely governs the performance of the magnetoresistive head is noise. Noise may include, for example, Barkhausen noise, that is generated by the domains in the free layer, and Johnson noise or shot noise that is caused by resistance. Barkhausen noise is noise that is generated due to the fact that the free layer magnetization possesses domains; thus, the Barkhausen noise can be suppressed by preventing generation of domains in the free layer by providing a domain control layer in the magnetoresistive effect head. Johnson noise or shot noise is dependent on resistance, so reduction of the head resistance is an effective means of reducing such noise. Magnetoresistive effect films having a high MR ratio and low magnetoresistance are currently being developed based on these concepts.
A sensor film requires processing by, for example, etching to define the element height direction and track width direction. In the case of the element height direction, increased fineness may be achieved by final mechanical processing; however, in the case of the track width direction, increased fineness must be achieved in the wafer step. For track width processing, a photolithographic technique or the like is employed, and due to the nature of this technique, it is desirable to perform processing on a planar face.
Chemical Mechanical Polishing (CMP) in the semiconductor field was developed with this end in view. Specifically, formation of a pattern of maximum fineness in the magnetic head takes place in this track width forming step and is preferably performed on a planar face. Consequently, as the processing of the sensor film in the wafer step, processing is desirably performed from the track width direction rather than the element height direction. However, in the ordinary manufacturing method, certain problems arise. This is because the hard bias film that is arranged at both ends in the track width direction of the sensor film is subjected to cutting processing in the same way as by processing in the element height direction. In this case, the volume of the hard bias film in the final slider condition depends on the dimension of the sensor film in the element height direction. Specifically, since fineness in the element height direction is being increased in the same way as increased fineness of the track width, only a dimension of the order of a few tens of nanometers (nm) will be left in the case of the element height direction also. In this case, regarding the element characteristics, possible diminution of the domain control effect due to the hard bias film may be a problem. This is because, in the case of magnetic films, such as hard bias films, magnetic stability is best guaranteed by a large volume; therefore, as the fineness is increased in the element height direction, the magnetic stability of the hard bias film itself is lowered, with the result that domain controllability is lowered. In the worst case, this may give rise to problems regarding generation of Barkhausen noise, etc.
In addition, severe effects may arise from other factors. Among these is the effect on the magnetic head SNR of noise (magnetic noise) produced by thermal fluctuations of free layer magnetization. While there has been an outstanding increase in the MR ratio in recent years in, for example, TMR heads, the accompanying improvement in the head SNR is saturated at a certain level. This is because, when the reproduction output is increased, the magnetic noise also increases proportionally, and the head SNR becomes saturated at a certain maximum value. In recent years, increase in fineness in the track width and element height direction has proceeded to the extent of the order of a few tens of nanometers, but, owing to the increased thermal effect, the problem of magnetic noise has become severe. Consequently, it is considered vital to reduce this magnetic noise in order to improve the head SNR as track fineness is increased in the future.